Unlike sound for television dramas, a motion picture normally records only the dialog (the spoken words) during production. All the other sounds are created in post production. This gives greater freedom when editing the dialog. Therefore, the natural sounds of the environment need to be added. This is especially true if the scene was recorded on a sound stage at the motion picture studio where the background sounds are never natural or realistic. The sound of the environment is the ambience and must be recorded in a real location. Even though ambience is very soft in the background, a scene will feel flat without it.
Ambience is also sometimes referred to as "room tone."